This invention relates to hydraulic fluids which maintain high wet equilibrium reflux boiling points on exposure to or contamination with moisture. The fluids are used in devices operated by fluid pressure, such as hydraulic brakes, clutches, fluid transmissions, shock absorbers, power steering and control devices for aircraft, ships, automobiles and other vehicles, artillery recoil mechanisms, door checks, jacks and other hydraulic devices adapted for transfer of mechanical energy.
A hydraulic fluid adapted for the above uses must meet a variety of requirements. It should be chemically stable, nearly nonvolatile, and of low flammability, and yet should remain a homogeneous flowable liquid at temperatures as low as -40.degree. C. Additionally, a hydraulic fluid is subject to moisture contamination which may arise because of the inherent hygroscopicity of the hydraulic fluid, from condensation of moisture from the air, or from physical leakage or defects in the hydraulic system that permits water to enter. The deleterious effects arising from moisture contamination of hydraulic fluids include lowering of boiling points, vapor locking, corrosion, hydrolysis, foaming, sludging, freezing, ice crystallization and the like.
Requirements to be met for a satisfactory hydraulic fluid particularly a hydraulic fluid used in motor vehicle hydraulic braking systems have been established by the United States National Highway Safety Bureau and are known generally as DOT 3, DOT 4, and DOT 5 (DOT referring to the Department of Transportation). Included therein are different requirements to be met by proposed hydraulic fluids such as: wet and dry equilibrium reflux boiling point (ERBP), wet and dry viscosity at -40.degree. C., chemical stability and rubber compatibility standards, etc. A hydraulic fluid satisfying the different testing standards is characterized as being a DOT 3-, DOT 4-, or DOT 5-fluid depending on the particular standard that is met or exceeded by the fluid. Details of this testing procedure are published in the Federal Motor Vehicle Safety Standard, 49 CFR .sctn.571.116.